Cap-sensitive watergel explosive composition production process

ABSTRACT

A watergel explosive composition is described which contains an oxidiser salt, a sensitiser, a thickener, a crosslinking agent, a fuel and less than 8%, by mass, of the composition of water. The explosive composition is cap sensitive in packages or cartridges having a diameter of about 26 mm or less at temperatures below about 5° C. when detonated with a standard number 6 strength detonator. The explosive composition does not require a supplementary sensitisor or a perchlorate salt to achieve this cap sensitivity. A method of producing the explosive composition by separately preparing a thickened aqueous phase and a dry phase, and allowing the thickened phase to stand for a period of time before the mixing of the two phases, as well as explosive paper cartridge containing the watergel explosive composition, are also described.

BACKGROUND OF THE INVENTION

This invention relates to an explosive composition of the type known asa watergel or a slurry-type composition and to a process formanufacturing this explosive composition.

Watergel or slurry explosives are widely used because they perform welland are relatively safe to make, store and use. The high water contentof watergel or slurry explosives has, however, necessitated that they becartridged in plastic packaging. This has drawbacks in that it haspresented difficulties inter alia with the proper loading of theexplosive into boreholes.

In South African Patent Application No. 94/2573, an improved watergel orslurry explosive is described which has a sufficiently low water contentthat it is dry to the touch and can therefore be packaged in standardwaxed paper cartridges of the type used for packing dynamite or othernitroglycerine sensitized explosives without causing the paper cartridgeto disintegrate. The hydrogel explosive compositions described in SouthAfrican Patent Application No. 94/2573 are capable of initiation by anumber six strength detonator in small diameter cartridges and they arethus said to be "cap sensitive in small diameter" cartridges.

The cap sensitivity of an explosive composition is a measure of thereliability of detonation of the explosive composition in the field. Thelower the temperature at which an explosive composition is capsensitive, the less prone it is to transient desensitisation and themore likely it is to detonate reliably in the field. Also, the smallerthe diameter of the cartridge, the more difficult it is to achieve capsensitivity at a low temperature. A perchlorate salt or pigmentaluminium is incorporated into the explosive compositions described inSouth African Patent Application No. 94/2573 to make them cap sensitivein small diameter at temperatures of 5° C. or less as these componentsare known to lower the temperature at which an explosive composition iscap sensitive.

While the inclusion of a perchlorate salt or pigment aluminium in anexplosive composition improves cap sensitivity of the composition andthus enhances its reliability, there are certain drawbacks to theinclusion of these components.

In certain countries, such as South Africa, perchlorate salts andpigment aluminium are expensive. Perchlorate salts are also veryreactive.

Therefore, while the inclusion of a perchlorate salt enhances the capsensitivity of a composition containing it, it also increases thefrictional and impact or shock sensitivity of the composition. Thismakes the composition relatively less safe to make, store, transport anduse. However, it has not been possible to do away with the perchloratesalt or pigment aluminium and still achieve low temperature capsensitivity in these low water watergel compositions when packaged insmall diameter.

SUMMARY OF THE INVENTION

According to the invention a watergel explosive composition is providedwhich contains an oxidiser salt, a sensitiser, a thickener, acrosslinking agent, a fuel and less than 8%, by mass, of thecomposition, of water and which is cap sensitive in packages having adiameter of about 26 mm or less at temperatures below about 5° C. whendetonated with a standard number six strength detonator, characterisedin that it does not require a supplementary sensitiser or a perchloratesalt to achieve this cap sensitivity. The water content is preferablyless than 6%, by mass, of the composition. More preferably, the watercontent is less than 5%, by mass, of the composition.

In the specification what is meant by a "standard number six strengthdetonator" is a 6D aluminium instantaneous electric detonatormanufactured by AECI Explosives Limited, or in internationalterminology, a "standard number six strength detonator" is a detonatorwhich has about 350 mg of PETN in the base charge.

The oxidiser salt is preferably a nitrate of ammonia or of an alkali oralkaline earth metal, or mixture of these. Although it is not necessaryto include a perchlorate salt in the low water watergel explosivecompositions of the invention, it may be useful in certain specialisedapplications to include a perchlorate salt as the, or as part of, theoxidiser salt component, although this would decrease the margin ofsafety of the composition.

More preferably, the oxidiser salt is unmilled ammonium nitrate prillson their own or in conjunction with nitrates of alkali or alkaline earthmetals, preferably sodium nitrate.

The sensitiser preferably consists of more than 50% by weight of one ormore water soluble compounds of oxygen balance more positive than -150%,selected from the salts of nitric, chloric and perchloric acid withacyclic nitrogen bases, having no more than two hydrogen atoms bonded toeach basic nitrogen atom and up to three carbon atoms per basic nitrogenatom, and the salts of nitric, chloric and perchloric acid with a phenylamine. The sensitiser may be a water soluble sensitiser and may be analkylamine nitrate or an alkanolamine nitrate. Preferred examples aremonomethylamine nitrate, ethanolamine nitrate, diethanolamine nitrate,triethanolamine nitrate, dimethylamine nitrate, hexamine nitrate,ethylenediamine nitrate, laurylamine nitrate and mixtures of these.

The preferred sensitiser is monomethylamine nitrate, either on its ownor in combination with other sensitisers.

The fuel may be a product of vegetable origin, such as a starch, flour,sawdust or woodmeal, rubber, coal or sugar or molasses or a vegetableoil.

Alternatively, it may be a product derived from crude oil. It may be asolid hydrocarbon, such as ground rubber and recycled plastic waste, ora liquid hydrocarbon such as glycol, waxes, ethylene glycol or otheralcohols, oils and waxes. It may also be a metallic fuel, such asaluminium, which is added to enhance the performance of the explosive.

The type of fuel used is not critical and is usually determined by costconsiderations. The quantity of fuel that is added is determined by theoxygen balance desired in the explosive composition.

The thickener may be a natural thickener, such as guar gum, accacia gumor galactomannin or starch. Alternatively, it may be a biosyntheticproduct such as xanthan gum. Further alternatively, it may be asynthetic polymer, such as polyacrylamide.

The watergel explosive composition may also contain a density reducingagent, which may be a solid void-containing material, such as perlite,glass microspheres or plastic microspheres or expanded polystyrene or achemical additive which is capable of generating gas in situ, forexample sodium nitrite.

According to another aspect of the invention a process for manufacturinga watergel explosive composition comprises the steps of:

preparing a thickened aqueous phase of water, at least a portion of awater soluble sensitiser, at least a portion of a thickener, a portionof an oxidiser salt, optionally a portion of a crosslinking agent andoptionally a portion of a fuel;

preparing a dry phase of the remaining oxidiser salt, any remainingthickener, any remaining crosslinking agent, any remaining fuel, and anyremaining water soluble sensitiser separately;

allowing the thickened aqueous phase to stand for a period of time;

mixing the two phases; and

reducing the density of the mixture by mixing the composition in such away so as to incorporate gas into it or by adding a density reducingagent.

The thickened aqueous phase may contain all of the crosslinking agent.Alternatively, the dry phase may contain all of the crosslinking agent.Further alternatively, the thickened phase may contain a portion of thecrosslinking agent and the dry phase may contain a portion of thecrosslinking agent.

The period of time for which the thickened phase is allowed to stand ispreferably at least about five minutes, more preferably at least aboutten minutes.

The oxidiser salt is preferably unmilled porous ammonium nitrate eitheron its own or in conjunction with sodium nitrate, potassium nitrate orcalcium nitrate.

The thickened aqueous phase preferably comprises between about 25% and80%, by mass, of the composition.

More preferably, it comprises more than about 30%, by mass, of thecomposition.

Most preferably, it comprises about 60%, by mass, of the composition.

According to yet another aspect of the invention an explosive cartridgecomprises a paper cartridge and a watergel explosive composition of theinvention contained within the paper cartridge. Preferably, theexplosive cartridge has a diameter below about 26 mm.

According to yet another aspect of the invention a method of cartridginga watergel explosive composition comprises the step of filling a papercartridge with the watergel explosive composition of the invention usinga cartridging machine of the type used to cartridge nitro-glycerinesensitised explosives.

Paper in this specification can be defined as any cellulosic materialwhich is substantially free of any plastics material.

DETAILED DESCRIPTION OF THE INVENTION

A watergel explosive composition which is reliably cap sensitive insmall diameter formulations (including those in packages or cartridgesbelow 26 mm) which can be cartridged in paper and which can be firedwith a standard number 6 strength detonator at temperatures of 5° C. orless without the need for the addition of a supplementary sensitiser ora perchlorate salt is described. This is entirely unexpected as untilnow it has not been possible to do away with the supplementarysensitiser and a perchlorate in watergel compositions of the low watertype and still to achieve a low temperature cap sensitivity in smalldiameter cartridges. The absence of a supplementary sensitiser and aperchlorate salt greatly reduces the friction and impact or shocksensitivity of the watergel compositions of the invention. It hastherefore been possible to produce a watergel composition with all thepackaging advantages of the watergels described in South African PatentApplication No. 94/2573 but with enhanced safety and cost advantages.

It has been possible to formulate these low temperature, small diametercap sensitive watergel compositions by using the unique productionprocess of the invention. The process involves a two step procedure ofproducing a thickened aqueous phase and a dry phase and allowing thethickened phase to stand for a certain period of time before adding thedry phase to it. The thickened phase should preferably be allowed tostand for at least about 5 minutes. It has been found that good resultsare obtained when the thickened phase is allowed to stand for about 10minutes. If the watergel explosive composition is prepared with anycrosslinker in the thickened phase, the time for which the thickenedphase is allowed to stand may have to be limited to less than about 60minutes as it could prove difficult to incorporate once crosslinking hastaken place.

Without wishing to be bound by theory, it is thought that the use of thetwo- step procedure for producing the watergel composition allows abetter re-growth of mixed crystals of water soluble sensitiser andoxidiser which are more reactive and thus which render the explosive capsensitive. The further step of allowing the thickened phase to standfurther promotes this crystal re-growth. When the thickened phase isthen mixed with the dry phase, the matrix of mixed crystals provides thesensitivity necessary to initiate the entire mass of the composition.

The thickened aqueous phase comprises water, at least a portion of awater soluble sensitiser, at least a portion of a thickener, a portionof an oxidiser salt, optionally a portion of a crosslinking agent andoptionally a portion of a fuel. The dry phase contains the remainingoxidiser salt, any remaining thickener, any remaining crosslinkingagent, any remaining fuel and any remaining water soluble sensitiser. Itwas also unexpectedly found that it is not necessary to prepare a clearsolution of oxidiser salt in the solution of water soluble sensitiser inthe thickened phase. It will be seen that in South African PatentApplication No. 94/2573 a clear solution was formed in the thickenedphase and to achieve this less than 30% of the water soluble componentsof the composition were incorporated into the thickened phase. It hasnow been found that as much as 60% or more of the water solublecomponents of the composition can be included in the thickened phase andthat an effective composition results even if there is undissolvedmaterial in this phase. It is to be noted that as long as the dry phasecontains at least some of the oxidiser salt, the other non-solublecomponents may be mixed into the thickened phase without adverselyaffecting the re-growth of mixed crystals of water soluble sensitiserand oxidiser.

The discovery that a low temperature cap sensitive composition can beformed without incorporating pigment aluminium, which is very expensive,or a perchlorate salt, which is also expensive and very reactive, is animportant one. Apart from the cost saving, the absence of perchloratesalts improves the impact or shock sensitivity of the composition. Thiscan be seen from the table set out below where four explosivecompositions, produced in the examples which follow, were tested byNaschem (A division of Denel (Pty) Limited) for impact sensitivity on aJulius Peters Impact Sensitivity Apparatus and for friction sensitivityon a Julius Peters Friction Sensitivity Apparatus.

    ______________________________________    Sample number  1       2        3     4    Example number 5       8        1     4    ______________________________________    Monomethylamine nitrate                   17.6%   17.6%    20.0% 16.0%    Ammonium nitrate                   67.6%   73.5%    71.6% 69.4%    Potassium perchlorate                   6.0%    0        0     0    Sodium perchlorate                   0       0        0     5.2%    Ground rubber  2.8%    1.9%     1.8%  3.0%    Organic Thickener                   1.6%    1.6%     1.6%  1.6%    Pigment aluminium                   0       1.0%     0     0    Water          4.4%    4.4%     5.0%  4.8%    ______________________________________

In a Julius Peters Impact Sensitivity Apparatus the sensitivity toimpact of explosives is tested by a Fallhammer method where each sampleis subjected to the action of a falling hammer of different masses fromdifferent heights.

The parameters to be determined are the height of fall at which asufficient amount of impact energy is transmitted to the sample for itto decompose or to explode. The sample is placed in a confinement devicewhich consists of two coaxial cylinders placed one on top of the otherand guided by a steel ring.

The impact sensitivity test results are summarised in table 1:

    ______________________________________           Height    Sample   1500 mm        2000 mm    ______________________________________    1        No ignition (20 trials)                            1 Ignition out of 6 trials    2        No ignition (10 trials)                            2 Ignitions out of 6 trials    3        --             No ignition (10 trials)    4        No ignition (10 trials)                            1 Ignition out of 6 trials    ______________________________________     All tests were conducted with a 5 kg hammer.

Sample 1 includes potassium perchlorate, Sample 2 includes pigmentaluminium and Sample 4 includes sodium perchlorate. At a height of 2000mm each of Samples 1, 2 and 4 were ignited. It will be seen that thesample containing pigment aluminium (Sample 2) was the most sensitive.The composition containing none of these (Sample 3) did not ignite, thusindicating the degree to which the removal of perchlorate salts and/orpigment aluminium decreases the impact sensitivity over a compositioncontaining it.

None of the samples were found to be friction sensitive.

In the United States, the relevant regulations preclude the use ofexplosives containing perchlorates in coalmines. The so-called"permissable" explosives, therefore, do not contain perchlorates. Theprocess of the invention for manufacturing a watergel explosivecomposition allows for the production of a composition (withoutperchlorate salts) suitable for use in mines with hazardous atmospheres,for example coalmines. By the addition of suitable halide salts to theexplosive compositions of the present invention, typically sodiumchloride (NaCl), potassium chloride (KCl), or ammonium chloride (NH₄Cl), suitable "permissable" or "permitted" explosives are obtained.

It has also been found that when using the process of the presentinvention it is possible to use unmilled ammonium nitrate prills andstill to achieve low temperature, small diameter cap sensitivity.Obviating the need to mill the ammonium nitrate eliminates a messy andtime consuming step in the production of watergel explosivecompositions.

When the density of the explosive composition is reduced by usingchemical gassing as the voiding means, a certain amount of gassing takesplace in the package or cartridge after cartridging has taken place.With a composition packed in plastic this is not normally a problemsince the package is strong enough to contain any gas generated, andthus any expansion in the volume of the composition, without rupturing.However, with a composition packed in paper, gassing in the package is areal problem since the gassing causes the composition to expand andincrease in volume. This pushes the crimps, which close the ends of thecartridge, open. This occurs because a paper cartridge is not positivelysealed in any way and it relies simply on the folds of paper to remainclosed.

When milled ammonium nitrate is used, and the composition is chemicallygassed, the problem is exacerbated because few interstices are presentin the composition before the gassing to accommodate the gas. Inproducing the low water paper packaged watergels of SA Patent 94/2573,where milled ammonium nitrate is used together with chemical gassing, itwas found that unless a degree of underfilling of the cartridge wasallowed, the crimps on at least one of the ends opened on standing for24 hours.

It has now been found with the present composition, where unmilledammonium nitrate prills are used, that by selecting a ratio of dry phaseto thickened phase, sufficient interstices are left between the prillsthat the composition can be packed into cartridges, without the need tounderfill the cartridges. The thickened phase then expands into theinterstices between the prills while the gassing process continues inthe cartridge, without the crimps at the end of the cartridge opening atall. The proportions of the various ingredients and in particular thepresence of sodium nitrate in the formulation dramatically affects thefinal rheology of the product and thus allows one to vary the texture ofthe product to suit the particular packaging equipment that it isintended to use.

The invention will now be described in more detail with reference to thefollowing examples.

EXAMPLE 1

In the mixer bowl of a Kenwood Chef fitted with a "K" blade (here calleda planetary mixer) was placed 550 parts of porous ammonium nitrateprills, 9 parts of guar gum, 27 parts of ground rubber, 1.5 parts ofadipic acid and 0.21 parts of potassium pyroantimonate. Theseingredients were thoroughly mixed and left in the mixer bowl under theplanetary mixer. (This mixture is the dry phase.)

Into a mixer bowl fitted with a propeller agitator (Hydolph stirrer) wasplaced 375 parts of monomethylamine nitrate solution of 80%concentration at 80° C. To this hot methylamine nitrate solution wasadded 1.8 parts of thiourea, the agitator was turned on and 434 parts ofporous ammonium nitrate was added followed, 30 seconds later, by amixture of 90 parts of porous ammonium nitrate and 15 parts of guar gum.

After mixing for about 2 minutes, the vortex from the agitatordisappeared and mixing was continued for another 30 seconds.

This mixture (which is the thickened phase) was set aside for 10minutes.

In a separate container was weighed 3 parts of a 15% sodium nitritesolution.

After the 10 minutes referred to above the thickened phase was added tothe dry phase and mixed in the planetary mixer at speed 2. After mixinghad proceeded for 30 seconds, the sodium nitrite solution was added andmixing was continued for a further 30 seconds. The resulting product waspackaged in 25 mm waxed paper cartridges. These cartridges fired at 5°C. with a No. 6 strength detonator. When tested on a Julius PetersImpact sensitivity Apparatus no detonations were recorded in 10 trialsfrom 2000 mm with a 5 kg hammer.

EXAMPLE 2

In this case exactly the same formulation was prepared as that used inExample 1 but the thickened phase was added to the dry phase immediatelyon completion of the thickened phase. In other words the thickened phasewas not allowed to stand for 10 minutes prior to addition of thethickened phase to the dry phase.

The rest of the mixing procedure was identical.

This formulation fired at room temperature (about 20° C.) but failed at5° C. in 38 mm diameter when tested with a No. 6 strength detonator. In25 mm diameter it failed at room temperature.

This example has been included for purposes of comparison. It will benoted that, unlike in Example 1, the prepared thickened phase was notallowed to stand for any period of time prior to the addition of thethickened phase to the dry phase. The resulting formulation lacked thecap sensitivity of the composition prepared in accordance with Example 1where the thickened phase was allowed to stand for 10 minutes.

EXAMPLE 3

Here exactly the same procedure was followed as that used for Example 2with the only change being that all the porous ammonium nitrate used wasmilled before use in an attempt to improve the sensitivity.

When tested in 38 mm diameter it was found that the product fired atroom temperature (about 20° C.) but failed at 5° C.

This example has also been included for purposes of comparison. It willbe noted that, unlike in Example 1 and Example 2, milled porous ammoniumnitrate, which should increase the cap sensitivity of the composition,was used. However, as the thickened phase was not allowed to stand forany period of time, the reduced cap sensitivity of the composition ofExample 2 could not be improved upon.

EXAMPLE 4

In this case the dry phase was made from 550 parts of milled porousammonium nitrate, 9 parts of guar gum, 45 parts of ground rubber, 1.5parts of adipic acid and 0.21 parts of potassium pyroantimonate. Thesewere mixed in the planetary mixer at speed 2 until they werehomogeneous.

The thickened phase was made from 300 parts of 80% monomethylaminesolution heated to 80° C. into which was dissolved 1.8 parts ofthiourea, 90 parts of sodium perchlorate monohydrate, 401 parts ofmilled porous ammonium nitrate. After mixing for 30 seconds, a mixtureconsisting of 90 parts of milled porous ammonium nitrate and 15 parts ofguar gum was added and mixing was continued at maximum speed until thevortex disappeared and then mixing was continued for a further 30seconds.

Immediately on completion of the thickened phase it was added to the dryphase and mixed for 30 seconds at speed 2 in the planetary mixer priorto the addition of 3 parts of 15% sodium nitrite solution, after whichmixing was continued for a further 30 seconds. The resulting productfired at 5° C. with a No. 6 strength detonator. When tested on a JuliusPeters Impact Sensitivity Apparatus 1 detonation was recorded in 6trials from 2000 mm with a 5 kg hammer.

EXAMPLE 5

This was an analogous mix to that of Example 4, the only change beingthe substitution of potassium perchlorate for the sodium perchloratemonohydrate used in Example 4. This mix also fired at 5° C. with a No. 6strength detonator. When tested on a Julius Peters Impact SensitivityApparatus 1 detonation was recorded in 6 trials from 2000 mm with a 5 kghammer.

Examples 4 and 5 have also been included for purposes of comparison.Unlike in Example 2, they include sodium perchlorate and potassiumperchlorate respectively. They also include milled porous ammoniumnitrate, which enhances the sensitivity of an explosive composition.However, in producing this composition, the thickened phase was notallowed to stand for any period of time. The resulting products were capsensitive despite the fact that the thickened aqueous phase was notallowed to stand. However, they were also impact sensitive. Theseexamples indicate the sensitising effect of perchlorate salts.

EXAMPLE 6

This was identical to Example 5 but made without milling the porousammonium nitrate. In this case the product fired at room temperature(about 20° C.) but failed at 10° C.

This example was also included for purposes of comparison. It is to benoted that by not milling the porous ammonium nitrate, the sensitivityof the explosive composition is reduced.

EXAMPLE 7

In this example only 45 parts of potassium perchlorate was used insteadof the 90 parts used in Examples 5 and 6. Again the porous ammoniumnitrate was not milled. In this case the product failed to fire at 20°C. with a No.6 strength detonator.

This example was also included for purposes of comparison. It is to benoted that by reducing the amount of the perchlorate salt that isincluded, the cap sensitivity of a watergel explosive composition issubstantially negatively affected.

EXAMPLE 8

This was very similar to Example 2 but with the addition of 1% pigmentaluminium with a surface area of 20000 cm² /g to the thickened phase.This formulation fired at room temperature (about 20° C.) with a No. 6strength detonator but failed at 5° C. When tested on a Julius PetersImpact Sensitivity Apparatus two detonations were recorded in six trialsfrom 2000 mm with a 5 kg hammer.

This example was also included for purposes of comparison. It is to benoted that the inclusion of pigment aluminium did not enhance the capsensitivity of the composition greatly but enhanced its impactsensitivity, and thus decreased the safety of the composition.

EXAMPLE 9

In this example the dry phase was made from 534 parts of porous ammoniumnitrate prills, 42 parts of ground rubber, 3 parts of guar gum, 1.5parts of adipic acid, and 0.3 parts of potassium pyroantimonate.

The thickened phase was prepared by heating 375 parts of monomethylaminenitrate solution of 80% concentration to 80° C. and adding 1.8 parts ofthiourea to this while stirring with a propeller agitator. To thisstirring solution was then added 345 parts of porous prilled ammoniumnitrate and a pre-mix of 180 parts of sodium nitrate and 21 parts ofguar gum. This mixture was mixed for three minutes and then allowed tostand for ten minutes before adding to the dry mix.

After combining the thickened phase and the dry phase in a planetarymixer they were mixed for 30 seconds, after which time 3 parts of a 15%sodium nitrite solution was added and mixing was continued for a further30 seconds.

This formulation was of a sufficiently low viscosity to be pumpable witha mono pump. The product was still compatible with paper. When packed incartridges it was found to be cap sensitive in 22 mm diameter at 5° C.

This demonstrates the influence of a change in ingredients on the finalrheology of the product. (It is clear when comparing this formulation tothat in Example 1 that the only substantive change is the substitutionof sodium nitrate for a portion of the ammonium nitrate.)

EXAMPLE 10

In this example the thickened phase was prepared as follows:

1250 parts of an 80% monomethylamine nitrate solution at a pH of between4.5 and 6.0 and containing 6 parts of thiourea was heated to 80° C.

This hot solution was placed in a mixer fitted with a propeller agitatorand the agitator was turned on. To this stirred solution was added 1000parts of unmilled porous ammonium nitrate followed by a premix,consisting 140 parts of ground rubber, 70 parts of guar gum (type DFS33, supplied by Hexachem) and 10 parts of a high protein guar (typeGempolym C, supplied by G. M. Associates).

After about 2 minutes of mixing (until the mix had thickened), thisthickened solution was placed in the mixer bowl of a planetary mixer andset aside for 10 minutes.

After 10 minutes the bowl was placed under the planetary mixer and themixer was turned on and a pre-mix of 1930 parts of unmilled porousammonium nitrate, 600 parts of sodium nitrate, 5 parts of adipic acidand 2 parts of potassium pyroantimonate (sold commercially as Liox by G.M. Associates) was added to the mixer. After mixing for one minute, 11parts of a 15% sodium nitrite solution was added and mixing wascontinued for another minute.

The product was then packed, by means of a MONO® pump, into waxed,spirally wound paper tubes.

Product made in this way is capable of initiation by a number 6 strengthdetonator at 5 ° C. in 25 mm cartridges.

This demonstrates that the order of addition of the sodium nitrate isnot critical to the final rheology of the product. This example alsodemonstrates that it is not necessary to have any thickener in the dryphase.

In all these examples 25 mm cartridges were used. In all these examplesno attempt was made first to produce a clear solution of ammoniumnitrate in the monomethylamine nitrate solution as is done in theexamples of South African Patent Application No. 94/2573. The quantityof ingredients used in the thickened phase and in the dry phase arechosen in order to provide a final product with the required rheology.

Examples 1, 9 and 10 clearly illustrate that using the unique process ofthe invention, a watergel explosive composition which is cap sensitivein small diameter is obtained even though unmilled oxidiser salt is usedand even though no perchlorate or pigment aluminium is added to thecomposition. What emerges from these examples is the importance ofallowing the thickened phase to stand for a period of time before theaddition of the dry phase to it. Without wishing to be bound by theory,it is assumed that this results in the formation of mixed crystals ofmonomethylamine nitrate and ammonium nitrate during the waiting time andthis results in the improved cap sensitivity of the compositions.

I claim:
 1. A process for manufacturing a cap sensitive watergelexplosive composition containing less than 6%, by mass of thecomposition, of water packaged in a cartridge comprising the stepsof:preparing a thickened aqueous phase of water, at least a portion of awater soluble sensitiser, at least a portion of a thickener and aportion of an oxidiser salt; preparing a dry phase of the remainingoxidiser salt, any remaining thickener, crosslinking agent, fuel, andany remaining water soluble sensitiser separately; allowing thethickened aqueous phase to stand for a period of time; mixing the twophases; reducing the density of the mixture by incorporating gas into itby adding a chemical density reducing agent; and filing the mixture intoa cartridge.
 2. A process according to claim 1, wherein the period oftime for which the thickened phase is allowed to stand is at least aboutfive minutes.
 3. A process according to claim 1, wherein the period oftime for which the thickened phase is allowed to stand is at least aboutten minutes.
 4. A process according to claim 1, wherein the thickenedaqueous phase comprises between about 25% and 80%, by mass, of thecomposition.
 5. A process according to claim 1, wherein the oxidisersalt is unmilled porous ammonium nitrate prills or a mixture of unmilledporous ammonium nitrate prills and a nitrate of an alkali or alkalineearth metal.
 6. A process according to claim 5, wherein the oxidisersalt is unmilled porous ammonium nitrate and sodium nitrate, potassiumnitrate or calcium nitrate.